Stent devices are used in the treatment of diseased human or animal bodily lumens to provide structural support to the lumens. Of particular relevance to the present invention are stent devices for supporting and holding open vasculature lumens of the human body.
In order for the stent devices to reach the treatment site, a delivery device is used that passes through vascular passageways, often traversing relatively long and tortuous paths. Surgical procedures for delivering stent devices to various locations in the body are known to one skilled in the art. Thin and flexible delivery devices are advantageous for reaching challenging treatment sites.
With the objective of a reduced profile delivery device, a stent device is generally provided in a radially reduced delivery state. The stent device may be crimped onto an inner support member and a restraining sheath is disposed over the stent device to maintain the stent device in the radially reduced configuration. The delivery device will then be fed through relevant passages of the vasculature with the stent device maintained in the delivery configuration by the restraining sheath. Once at the treatment site, the restraining sheath is operated upon so that the stent device can radially expand to a deployed configuration for supporting and holding open a portion of a vein or artery or other bodily lumen.
Stent devices have a structural framework for supporting a diseased vasculature. Different arrangements for the structural framework are found throughout the relevant art. Often, the stent device will be formed from a seamless tubular work-piece into an elongate device having a number of axially spaced rings of stent material with each ring formed of zigzagging struts and with each ring connected to another by several circumferentially distributed connector struts. The inner major surface, the outer major surface or both may be covered to make the stent device liquid impermeable, which is then often named a stent graft. The framework may, however, be uncovered, which is then often named a bare stent.
Stent devices may be balloon expandable or they may be self-expandable. Balloon expandable stents may be radially expanded into the deployed configuration by action of a circumferential balloon disposed between the inner member and the stent device. The balloon is inflated, usually by injection of saline solution, thereby expanding the stent device into the deployed configuration. The present applicant has particular experience with self-expandable stent devices. These can be made of shape memory materials, such as the shape memory Nickel Titanium alloy known as Nitinol. Nitinol can be constructed into a desired deployed configuration and then crimped into the reduced profile delivery configuration. As long as the Nitinol stent device is kept below a solid phase transition temperature, the deformation of the reduced profile delivery configuration will be kept. Once raised above the phase transition temperature, such as at a body temperature, the Nitinol stent device will return to its original, radially expanded delivery configuration when it is allowed to do so, that is when it is released by the restraining sheath.
The prior art is replete with examples of ways to release the stent device from the overlaying restraining sheath. Two examples are prevalent.
One example is to slide the restraining sheath over the stent device in order to expose the stent device. The frictional forces between the stent device and the restraining sheath can be great, particularly as the stent device becomes more highly crimped, which causes increased radial forces straining on the sheath as it slides over the stent device. Similarly, a longer stent device will have greater frictional contact with the restraining sheath. This latter contributor to the frictional forces between the restraining sheath and the stent device is important in applications where stent devices have lengths greater than 200 mm and up to 300 mm or more. A pull member, extending proximally to a handle of the delivery device, is generally used to retract the outer sheath by pulling on it to move the outer sheath relative to the stent device to expose the stent device. One example of this form of release of the stent device can be found from WO 2004/096091. The high frictional forces involved in dragging the restraining sheath over the stent device can be damaging to the stent device and can make the pull force required to move the restraining sheath undesirably high.
Another example of a general way to release a stent device is to tear the restraining sheath axially along the stent device, thereby releasing the stent device to radially expand. One or more perforation lines can be provided in combination with a pull member running adjacent the perforation line or lines. As the pull member is pulled, the restraining sheath is torn at the perforation line to radially release the stent device. We found U.S. Pat. No. 5,246,452, US 2008/0243224 and WO 99/53865 to be interesting example disclosures of such technology. Also known is the tearing of the restraining sheath by using a pull member to cut therethrough, such as with a wire pull member that is able to slice through the restraining sheath. An interesting example of this kind of technology is disclosed in EP 0732087. One difficulty with such release mechanisms is that all or a part of the restraining sheath remains between the stent device and a wall of the bodily lumen. This may not be desirable in terms of properly engaging the stent device with the wall of the bodily lumen and may be disadvantageous in so far as it only allows certain materials to be used for the restraining sheath, which are long-term biocompatible.
Accordingly, it is an objective of the present invention to provide a mechanism for releasing a stent device from a restraining sheath that removes the restraining sheath from between the stent device and a wall of the vascular lumen and also allows a relatively low pulling force in carrying out the stent device release.